Fellow shell trivia enthusiast, hi!

    $ bash -c 'realpath /dev/stdin <<< small'
    /proc/79631/fd/pipe:[5282632]
    $ bash -c 'realpath /dev/stdin <<< $(printf "%65536s")'
    /tmp/sh-thd.yZ2L6p (deleted)

You can see the cutover on buffer size from my install of bash 5.2.15. Anything under 64k of here-string will still use a pipe.

In case you weren't aware, you can also force a "real" file with zsh process substitution by using =(…)¹. It can be useful when the tool you're using doesn't behave correctly with <(…), if it wishes to seek across it for example. Sadly, =(…) isn't supported in bash [yet?].

¹ In your case it still wouldn't hit a disk as it is in /tmp, but it at least becomes seekable.